The invention relates to cement kilns and more particularly a system for feeding waste derived fuel to a calciner kiln in a continuous manner during the manufacturing of cement.
Cement kilns are massive cylindrical structures, lined with refractory brick, into which a lime-bearing material and fuel such as coal or gas is fed. There is a range of different kiln designs but all rely on the same basic processesxe2x80x94raw feed material passing through the kiln is heated to very high temperatures by the burning of fuel and is transformed chemically and physically into a grey pebble-like material called clinker. Clinker is ground to produce cement. The three broad processes of cement manufacturing are: (1) raw milling, the preparation of the kiln feed material; (2) calcining and burning or clinkering, the material conversion process that takes place within the cement kiln and associated equipment; and (3) finish milling, the grinding of clinker to produce cement with the addition of gypsum.
The main raw material for the manufacture of clinker is a lime-bearing material. Ideally, this material is mined or dredged from a location near the plant to minimize transport and handling costs. Limestone is the most common material used, although other calcareous material (calcium carbonate) can be used. A large percentage, by weight, of the lime-bearing material is lost as carbon dioxide in the manufacturing process. Raw materials are combined to form a raw mix, and the proportions are governed by the chemical properties of the materials to be used and the desired properties of the clinker that is produced. The raw materials for a dry process kiln are mixed to produce an aerated dry raw material.
There are four stages to the process of the burning of the raw mix in a calciner kiln to form clinker; evaporation and preheating, calcining, clinkering and cooling. Evaporation and preheating remove moisture and raise the temperature of the raw mix preparatory to calcining. Calcining takes place at 800-900 C (gas temperature) and breaks the calcium carbonate down into calcium oxide and carbon dioxide which is evolved in the process. Clinkering completes the calcination stage and fuses the calcined raw mix into hard nodules resembling small grey pebbles. Kiln temperatures in the burning zone range from 1350-1450 C (material temperature). Retention times in the calciner range from 4 to 6 seconds. Retention times in the kiln is approximately 20 minutes.
Clinkering is critical to the quality of cement and requires accurate control of the energy input. Insufficient heat will cause the clinker to be underburnt, containing unconverted lime, and reducing the hydration (setting and hardening) properties of the resulting cement. Excess heat will shorten the life of the refractory bricks lining the kiln, may damage the kiln shell and diminish product reactivity. The high temperatures required for burning of the raw mix mean that the process is energy intensive. Electrical energy is required for the raw milling and mixing, burning, and finish milling. The largest energy demand is for fuel for burning of the raw mix. This has been the focus of technological developments, and the need to reduce energy in the cement manufacturing process remains a problem.
Rotary kilns are large steel tubes typically over 50 meters in length and up to 7 meters in diameter. They are slightly inclined to the horizontal and are slowly rotated at about 1 to 4 revolutions per minute. Raw mix is fed into the kiln at the back (the upper end of the kiln), and gravity and the rotation of the kiln allows the mix to flow down the kiln at a uniform rate through the burning zone. Clinker is formed in the burning zone and flows out of the front of the kiln (the lower end of the kiln) where it is cooled by blowing air through the clinker bed, and the heated air is used for combustion. The primary fuel is introduced and burnt at the front of the kiln. The flame is drawn up the kiln to the burning zone where the heat intensity is highest and fusion of chemicals in the raw mix takes place. Hot combustion gases continue to flow up the kiln and exit from the back end. Opportunities for recovering heat from kiln exhaust gases have made incremental improvements in lowering the energy demand of the kiln.
Suspension preheaters comprising a series of vertically aligned cyclones attached to the back end of the kiln have been advanced to reduce energy requirements in calciner kilns. Hot kiln exhaust gases flow up through the cyclones, and the raw mix is fed in at the top of the cyclones. As the raw mix travels down through the cyclones, it is heated by the exhaust gases, preheating the mix and initiating calcination.
The cement manufacturing process is carried out in wet kilns or calciner kilns using a rotary kiln. The calciner kiln process is carried out in a kiln that is commonly referred to as a calciner kiln. Both the wet kiln and the calciner kiln have utilized waste materials in order to reduce energy demands. In the wet kiln process, both liquid and solid waste materials have been introduced in various ways at the burner end of the kiln. Typically the liquid hazardous waste is injected into the burner by way of a nozzle. The hazardous solid waste materials are typically pneumatically fed into the burner end of the kiln. In the calciner kiln, liquid waste fuels are burned by introducing the liquid waste fuel mainly by injecting the fuel through a nozzle into the calciner or the kiln. One process for burning solid waste material is the hot disk process where solid waste fuel is placed on a rotating disk and combusted, and the ashes are removed while the disk rotates. The air passing through the hot disk is drawn into the calciner for heat and further combustion. Another process is a gasifier process, which is basically a pyrolysis process, where the solid waste material is roasted to evaporate the volatile ingredients which are then combusted in the calciner. There is also a batch process using calciner kilns where packages or containers of solid waste fuel are delivered to the feed shelf and combusted in the kiln. The batch process is generally not cost efficient because the waste material has to be processed into the containers which adds an additional cost to the process and the containers are combusted in a intermittent manner rather than a continuous manner.
U.S. Pat. No. 6,470,812 B1 discloses a method and apparatus for recovering energy from solid wastes by burning in the tertiary air duct of a calciner kiln. The patent includes a good discussion of prior patented inventions relating to reducing energy demands of cement kilns by burning solid wastes.
Another recent patent, U.S. Pat. No. 6,345,981 B1 discloses a ram feed for feeding solid wastes directly into the riser duct of a calciner kiln. U.S. Pat. No. 5,555,823 discloses an auger feed for feeding solid wastes directly into the calcining zone of a preheater kiln. U.S. Pat. No. 5,349,910 discloses a decomposition chamber for incinerating solid wastes using heat from preheated or calcined raw meal.
While advances have been made in reducing the energy demands and burning solid waste materials in calciner kilns, considerable attention still needs to be given to improving complete burnout of solid waste fuels with minimal residuals and recovering more of the heat of combustion.
Accordingly, an object of the present invention is to provide a system for feeding solid waste derived fuel (SWDF) material to a calciner in a consistent manner to provide kiln stability and more efficient operation.
Another object of the invention is to provide a system and method for feeding (SWDF) material to the calciner of a calciner kiln wherein the feeding of the material is continuous and, therefore, consistently provided to the combustion process.
Another object of the present invention is to provide a more stable calciner kiln process wherein (SWDF) materials are metered and delivered in consistent blends in a continuous manner during the calcination and kiln process.
Another object of the invention is to provide a feed system for feeding shredded SWDF material, either in loose bulk or containerized form, directly into the calciner along with raw kiln meal where the SWDF is burned with other fuels such as oil, gas, coal, petroleum coke, and liquid waste derived fuel during calcination.
The above objectives are accomplished according to the invention by providing a continuous feed system for feeding SWDF material directly to a calciner along with raw kiln material where the SWDF material is burned and the kiln material is calcinated at a controlled temperature in a high oxygen atmosphere.
The calciner kiln, for producing cement from raw kiln material, includes a preheater having a plurality of cyclone stages for heating the raw kiln material, and a calciner having a calciner combustion chamber and a calciner duct for calcining the raw kiln material. A rotary cement kiln has a charge end and a discharge end for producing clinker from the calcined raw kiln material. A riser duct is disposed between an in-line duct of the calciner duct and the charge end of the rotary kiln. The riser duct and in-line calciner duct are in-line with a flow of hot kiln gas from the charge end of the rotary kiln. A raw kiln material feed supplies raw kiln material, which may be preheated, into the combustion chamber. A waste material feed system continuously feeds solid waste derived fuel (SWDF) material into the combustion chamber. The calciner combustion chamber has a controlled environment for burning the SWDF in the presence of the raw kiln material for calcining the raw kiln material. A connector duct connects the calciner combustion chamber and calciner duct for delivering the calcined kiln material, heat of combustion, and any ashes of non-combustible material to the calciner duct at an entry zone above the riser duct.
Advantageously, the combustion chamber includes a burner producing a burner flame to control the temperature in the calciner combustion chamber and burning of the SWDF material. An air inlet introduces combustion air into the combustion chamber having a high oxygen content generally equal to that of ambient, and a fuel control regulates the feed of a control fuel to the burner. The control fuel provides a more spontaneous combustion than the SWDF material to provide a more effective control over the temperature in the calciner combustion chamber. An intermediary cyclone stage collects the raw kiln material, and the raw kiln material feed supplies the raw kiln material to a feed inlet of the combustion chamber. The preheater includes a bottom separation cyclone for collecting calcined raw material and feeding the calcined raw material to the rotary kiln. Preferably, a control element splits the feed of raw kiln material between the calciner combustion chamber and vertical calciner duct, and the waste material feed system includes a feed control for feeding the SWDF material to the calciner combustion chamber at a controlled amount.
In an advantageous embodiment, the feed system for feeding SWDF material to the combustion chamber includes a feed hopper for accumulating the SWDF material, and a auger feed for feeding the SWDF material from the hopper to the combustion chamber. The auger feed includes a compression section and an expansion section wherein a compressed plug of the SWDF material is produced in the compression section and is fed to the expansion section for release into the combustion chamber in the form of loose shredded material. Preferably, a first shutoff gate is disposed between the compression section and the expansion section of the auger feed; and a second shutoff gate is disposed between the expansion section and a feed outlet of the auger feed. The shutoff gates are operable to open and close the feed of SWFD material through the auger feed. The first shutoff gate has an open position to allow feed of SWDF material through the auger feed and a closed position to block the feed of SWDF material to the expansion section. The second shutoff gate has an open position to allow feed of SWDF material to the auger feed outlet and a closed position to block the feed of SWDF material to the auger feed outlet. A controller may be provided for controlling the open and closed positions of the first and second shutoff gates. The controller positions the first and second shutoff gates in their closed positions in response to a controller input to shut down the SWDF feed system so that hot combustion gas and fire from the combustion chamber are not allowed to flow backward through the feed system. The controller operates the auger in a reverse operation prior to moving the first and second shutoff gates to the closed positions to release the compression of the compressed plug in the compression section of the auger feed. A feed conveyor feeds the SWDF material to the auger feed hopper as loose shredded material. Preferably, the conveyor includes a housing enclosing the feed conveyor; and including means for filling the enclosure with an inert gas to prevent combustion of the conveyer SWDF material. A conveyor controller controls the feed rate of SWDF material to the feed hopper so that a desired fill of SWDF material is maintained in the feed hopper. In another aspect, a fuel conditioner bin contains fuel conditioner; and a bin outlet feeds the fuel conditioner to the feed hopper of the auger feed to mix with the SWDF material enhancing the flowability of the SWDF material.
The objectives of the invention are achieved according to the invention by a method for feeding and burning solid waste derived fuel (SWDF) material during the calcination of raw cement material in the manufacture of clinker, which clinker is manufactured by a process by which raw cement material is preheated in a preheater, calcined in a calciner, burned into clinker in a rotary kiln, whereafter the clinker is eventually cooled in a cooler. More particularly, the method for feeding and burning the SWDF material during the calcining process includes; introducing the SWDF material into a combustion chamber; burning the SWDF material in the combustion chamber in a high oxygen atmosphere at a controlled temperature and fuel feed to achieve a generally complete burnout. Combustion air is fed to the combustion chamber as tertiary air from the clinker cooler. Next, the combustion gases are fed from the combustion chamber into a riser duct of the rotary kiln wherein the combustion gases mix with hot kiln gases flowing from the rotary kiln so that said hot combustion gases and hot kiln gases circulate through the calciner to complete any necessary burnout and calcination of the raw kiln material. Preferably, the method includes feeding the SWDF material to a feed hopper, and feeding the SWDF material from the feed hopper to an auger feed having a compression section and an expansion section. The SWDF material is compressed to form a compressed plug in the compression section which is released in the expansion section so that loose shredded SWDF material falls into the combustion chamber. The flow of SWDF material through said auger feed is controlled by varying the speed of the auger drive. By placing a first shutoff gate at an end of the compression section and placing a second shutoff gate near the outlet of the auger feed, the first and second shutoff gates can be closed to block the flow of hot gases and fire from the combustion chamber into the feed hopper during shut down of the combustion chamber. The compressed plug acts as an air seal so hot air does flow backwards during operation and false ambient air is not sucked into the combustion chamber.